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BS - Gastrointestinal System pt.1 > The Stomach > Flashcards

The Stomach Flashcards

1
Q

What are the main GI tract functions applicable to the stomach?

A

It is mainly involved in mechanical processing, digestion and secretion

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2
Q

When may ingestion be a function of the stomach?

A

If the stomach is used directly as an input mechanism through percutaneous endoscopic gastrostomy

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3
Q

What is percutaneous endoscopic gastrostomy?

A

A tube passes through the anterior abdominal wall into the stomach

It is anchored by a balloon and is used for feeding directly

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4
Q

When may the stomach be a route of excretion or absorption?

A

It is only a route of excretion through vomiting

Lipid soluble substances can occasionally be absorbed through the wall of the stomach

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5
Q

Where does the stomach begin and terminate?

A

The oesophagus almost immediately enters the stomach after passing through the diaphragm

It terminates at the duodenum

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6
Q

What is the interior surface of the stomach comprised of?

A

A mucosal layer of epithelium

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7
Q

What are rugae?

A

Folds of epithelium in the stomach

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8
Q

What is the purpose of rugae?

A

As the stomach fills with food, the epithelium can flatten out

There will not be a rise in tension as the stomach fills, allowing it to have a wide range of volumes

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9
Q

What is the muscular layer of the stomach?

A

This is the layer outside of the epithelium

It consists of longitudinal, circular and oblique muscle layers

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10
Q

What does contraction of the oblique muscle layer help with?

A

It helps to mix and liquify the food

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11
Q

What is the lesser omentum?

Where does it start and terminate?

A

It is a mesentery formed on the lesser curvature of the stomach

It runs from the lesser curvature to the porta hepatis (entrance to the liver)

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12
Q

What is the greater omentum?

How many sheets of peritoneum is it formed from?

A

It is a double sheet of peritoneum that descends from the greater curvature

It then picks up another double sheet of mesentery from the colon to form the greater omentum

It consists of 4 layers of peritoneum

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13
Q

In which region of the stomach is the muscle layer much thicker and stronger?

Why?

A

In the pyloric-antral region

This allows food to be squeezed back and forth within this region until it is small enough to pass through the pyloric sphincter

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14
Q

What is the main function of the upper part of the stomach?

A

It does have peristaltic waves, but its main function is to be able to relax

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15
Q

What is the luminal secretion of the LES and cardia?

What are the functions of this region?

A

Mucus and HCO3-

The main functions are:

  1. prevention of reflux
  2. entry of food
  3. regulation of belching
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16
Q

What is the luminal secretion of the fundus and body?

A
  1. H+
  2. intrinsic factor
  3. mucus
  4. HCO3-
  5. pepsinogens
  6. lipase
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17
Q

What are the main functions of the fundus and body?

A
  1. acts as a reservoir

2. provides tonic force during emptying

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18
Q

What is the luminal secretion of the antrum and pylorus?

What are the main functions of this region?

A

Mucus and HCO3-

This region is involved in:

  1. mixing, grinding and sieving of food
  2. regulation of gastric emptying
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19
Q

What is the lower oesophageal sphincter (LES)?

What is its function?

A

It is at the lower part of the oesophagus

It constricts to control flow into and out of the stomach

It helps to prevent acid refluxing back up into the stomach

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20
Q

Why is the stomach acidic even though the fundus and body secrete bicarbonate?

A

Both acid and bicarbonate are secreted

The mucus traps the bicarbonate close to the surface, whilst the acid is released into the lumen

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21
Q

When will the pyloric sphincter allow contents of the stomach to pass through into the duodenum?

A

Once the foodstuffs have been ground down to small enough molecules

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22
Q

How does the pyloric sphincter regulate emptying of the stomach?

Why is this important?

A

It controls the rate at which substances enter into the duodenum

This gives time for the small bowel to process the contents

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23
Q

What are the 4 main regions of the stomach?

A
  1. cardia
  2. fundus
  3. body
  4. pylorus
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24
Q

What are the different parts within the pylorus?

A

The pyloric antrum is the wider end of the pylorus which connects to the body

The narrower end is the pyloric canal, which connects to the duodenum

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25
Q

What are the fundus and body of the stomach responsible for?

A
  1. accepting and holding food
  2. relaxation
  3. accommodation
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26
Q

What is the antrum and pylorus responsible for?

A

The antrum is responsible for churning and grinding food

The antrum and the pylorus are responsible for the emptying of the stomach

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27
Q

What is accommodation?

What does it require?

A

It is an active relaxation process that is neurally controlled

It requires a vago-vagal reflex

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28
Q

What does accommodation depend on?

A

Stretch sensors in the stomach which signal via the vagus nerve

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29
Q

What happens when descending signals via the vagus nerve act on the enteric nervous system?

A

It results in relaxation and dilatation of the fundus and body of the stomach

This allows up to 1 litre of food to be stored in the stomach with very little rise in pressure

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30
Q

What is the motion of the fundus and body like during digestion?

A

They are relatively still

Occasionally peristaltic waves will squeeze off another “bite” and pass it to the antral pump

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31
Q

How can the antral pump adapt to the type of food in the stomach?

A

Liquid is allowed to pass through the pyloric sphincter

Thicker material is churned and ground until it is smooth enough to be passed into the duodenum

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32
Q

What happens in vagotomy (vagus nerve cut) as the stomach begins to fill?

A

The brain has no way of knowing the stomach is filling so the pressure progressively rises

There is no accommodation

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33
Q

How does the amount of fluid the stomach can hold vary in people with and without a vagotomy?

A

In normal people, much more fluid can enter the stomach without the pressure rising

This is because the stomach expands as it fills due to accommodation via the stretch receptors

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34
Q

What size must particles be to be unable to pass through the pyloric sphincter?

A

Larger than 1 - 2 mm

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35
Q

What does the duodenum sense and what hormones will it secrete in response?

A

It senses the delivery of acids, amino acids and lipids

It releases CCK, secretin and GIP

This decreases gastric motility and emptying

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36
Q

How is the nervous system involved in controlling gastric emptying?

A

The vagus nerve innervates the duodenum and helps regulate gastric emptying

The enteric nervous system also controls this, and includes feedback from the duodenum

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37
Q

How does the rate of gastric emptying vary after eating a saline and acidic meal?

A

The stomach empties much faster after a saline meal

If an acidic meal is consumed, the acid signals as it reaches the duodenum

This slows gastric emptying to allow more time for processing

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38
Q

What types of meals take the longest to process?

A

Oleate (oil) meals

The duodenum senses oily foods and slows down gastric emptying to give it longer to process the food

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39
Q

Where are the gastric glands found?

A

They are large and buried deep in the stomach

40
Q

what are the surface epithelial cells that continue in to the neck of the gastric gland?

A

Mucous cells

They secrete mucous which helps to protect the cells from acid and enzymes

41
Q

What else will the surface epithelial cells entering into the gastric gland secrete?

A

Bicarbonate

This sits in the mucous layer and stops the acidic pH from damaging the surface of cells

42
Q

What are gastric pits and glands mostly classified as?

What does this mean?

A

They are mainly oxyntic

These are epithelial cells which secrete hydrochloric acid and intrinsic factor

43
Q

Where are oxyntic cells found?

A

In the gastric glands found in the lining of the fundus and in the cardia of the stomach

(also called parietal cells)

44
Q

What is the role of G cells ?

Where are they mostly found?

A

They secrete gastrin into the bloodstream

They are mainly found in the antral/pyloric glands

45
Q

Why are G cells referred to as ‘signalling cells’?

A

They sense what is occurring in the stomach and can release gastrin accordingly

They signal to other parts of the stomach, the duodenum and pancreas

46
Q

What are Chief cells and what is their role?

A

They are also called peptic cells

They secrete pepsinogen into the stomach

47
Q

What are the 2 forms of gastrin?

What is similar about them?

A

G17 (17 amino acids)

G34 (34 amino acids)

The C-terminal 17 amino acids of G34 are the same as G17

48
Q

What is the difference between G17 and G34?

A

G17 is the main form of gastrin secreted from the antrum

G34 is the main form of gastrin secreted from the duodenum

49
Q

What is the main gastrin receptor?

What is meant by gastrin being homologous with CCK-PZ?

A

The main gastrin receptor is CCK-B

Gastrin is homologous with CCK-PZ as the C-terminal 5 amino acids are the same

50
Q

What 2 things can stimulate release of gastrin?

A
  1. G cells detecting the presence of amino acids and proteins in the lumen
  2. parasympathetic input
51
Q

What is the parasympathetic input like which stimulates gastrin release?

A

It is cephalic and in response to gastric stretch

Vagal stimulants being to initiate gastrin release as part of the cephalic phase - before food is actually eaten

52
Q

What is gastrin release inhibited by?

What type of mechanism is this?

A

Luminal H+

This is a negative feedback mechanism

As acid begins to accumulate in the lumen, it inhibits gastrin production to prevent more acid being released

53
Q

What happens to the stomach if there is no gastrin present?

A

The stomach eventually atrophies

54
Q

What are the main actions of gastrin on the stomach?

A
  1. stimulate acid secretion

2. promote mucosal growth

55
Q

What is somatostatin?

A

a peptide hormone which exists in 14 and 28 amino acid forms

56
Q

Where is somatostatin released from?

By which mechanisms does it work?

A

the D cells of the stomach, duodenum and pancreas

It mainly acts in a paracrine fashion (but can also be endocrine)

57
Q

What will stimulate and inhibit the release of somatostatin?

A

Release is stimulated by luminal H+

The release is inhibited by acetylcholine (parasympathetic activity)

58
Q

What is the function of somatostatin?

A

It acts on G cells to inhibit the release of gastrin

It also inhibits the release of CCK and secretin in the duodenum

59
Q

What do parietal (oxyntic) cells store in their cytoplasm?

A

They store many tubulovesicles within their cytoplasm

The proton pumps which secrete acid sit in the vesicles

60
Q

Why is there no acid secretion at rest, referencing the oxyntic cells?

A

The tubulovesicles in the oxyntic cells are not embedded in the membrane

61
Q

What happens to the tubulovesicles when gastrin and acetylcholine stimulate acid secretion?

A

The vesicles fuse into the indentations in the cell membrane to form channels called canaliculi

The proton pumps now sitting in the membrane can secrete acid

62
Q

What 2 hormones stimulate gastric acid secretion?

What hormone inhibits gastric acid secretion?

A

gastrin and acetylcholine

inhibited by somatostatin

63
Q

By which pathways do gastrin, ACh and somatostatin act?

How are they mediated?

A

They work by both direct and indirect pathways

The indirect effect is mediated by histamine

64
Q

What is the active pump involved in acid secretion and how does it work?

A

The H+/K+ ATPase

It ejects protons from the cell and picks up K+ ions

65
Q

What is the source of protons for the H+/K+ ATPase?

A

Carbonic anhydrase converts carbon dioxide and water into bicarbonate ions and H+

The bicarbonate ions are pushed out into the circulation

The H+ are pushed into the lumen of the stomach by the H+/K+ ATPase

66
Q

What is meant by the “alkaline tide”?

What is it coupled with and what does it result in?

A

The movement of bicarbonate ions into the circulation whenever substances are eaten

It couples movement of H+ ions into the stomach lumen

It causes the blood pH to rise slightly

67
Q

What drives the process behind the action of the H+/K+ ATPase?

A

Interactions with the gastrin receptor (GPCR) and activity from the parasympathetic nervous system

They drive the process through a cascade which leads to activation of the H+/K+ ATPase

68
Q

What type of cells mediate the indirect effect of gastrin, ACh and somatostatin?

A

enterochromaffin-like cells (ECL cells)

they sit within the wall of the stomach but have a nerve cell-like function

69
Q

What do enterochromaffin-like cells detect?

What is the outcome?

A

They detect the presence of gastrin and the activity of the parasympathetic nervous system

They release histamine

Histamine acts through a histamine receptor to trigger the same cascade

70
Q

How does somatostatin affect ECL cells?

A

It inhibits them

71
Q

What is the effect of prostaglandin E2 on acid production?

A

It acts through a GPCR, but will suppress acid production

72
Q

Why do drugs, such as aspirin and ibuprofen, increase the risk of gastric ulcers?

A

These drugs prevent the action of prostaglandin E2

Gastric acid production is not suppressed which increases the risk of gastric ulcers

73
Q

Why is the secretion of bicarbonate and mucus important?

A

It protects the epithelium of the stomach from the acidic environment

74
Q

What is the pH immediately outside of the gastric epithelial cells?

Why?

A

pH = 7 compared to pH 2 in gastric juice

Goblet cells produce a layer of mucous which sticks close to the epithelial cells

Bicarbonate that is secreted becomes trapped in that mucus layer

75
Q

How may stomach ulcers be caused by H. pylori infection?

A

H. pylori resides in the mucus and makes the enzyme urease

Urease breaks down the mucus layer

This allows acid to get through and damage the epithelial cells

76
Q

What is intrinsic factor and where is it secreted from?

A

It is a 55 kDa glycoprotein

It is secreted from parietal cells into the stomach

77
Q

What is the action of intrinsic factor in the stomach?

A

It is secreted into the lumen with the food but is inactive in the secreted form

It needs to be processed in the small intestine in order to have an effect

78
Q

What will intrinsic factor bind to in the small intestine?

Why is this important?

A

It binds to cobalamin (vitamin B12) in the small intestine

This is essential for cobalamin uptake in the ileum

79
Q

What happens to intrinsic factor in the distal part of the ileum?

A

There are receptors in the distal part of the ileum that recognise intrinsic factor

They internalise it, and pick up the vitamin B12 in the process

80
Q

What are chief cells and what do they secrete?

What is the main stimulant of the chief cells?

A

Chief cells are involved in pepsinogen secretion

There are neurones and interneurons coming into the chief cells, carrying parasympathetic input

81
Q

How does gastrin have an effect on the chief cells?

A

Gastrin stimulates the chief cells to release pepsinogen

The G cells detect amino acids, leading to gastrin secretion

Gastrin enters the circulation and exerts an effect on the chief cells

82
Q

What mediates the effect of gastrin on the chief cells?

How does secretin affect the chief cells?

A

The effect of gastrin is mediated by CCK-A receptors

Secretin from the duodenum has a much smaller role in pepsinogen secretion

83
Q

What triggers the cleavage of pepsinogen to pepsin?

What happens to pepsin once it is activated?

A

Acid secretion from the parietal cells in response to gastrin

Once activated, pepsin is self-perpetuating and can cleave more molecules of pepsinogen

84
Q

What are pepsins?

What are they secreted in response to?

A

They are a family of proteases which are secreted from chief and mucus cells

They are secreted in response to acetylcholine and H+

85
Q

What hormones have minor effects on pepsin secretion?

A

Secretin, CCK and gastrin

CCK and gastrin work via the CCK-A receptor

86
Q

How can pepsinogens be cleaved?

A

They are cleaved spontaneously at low pH (below 5)

Cleavage occurs more quickly is pH is less than 3

Pepsinogens may be cleaved by pepsin - this is autolysis

87
Q

What is meant by pepsins being endoproteases?

A

They cut within the peptide chain, rather than at the terminal amino acids

They break the protein into smaller peptides, but not into individual amino acids

88
Q

What types of proteins to pepsins primarily target?

A

Amino acids with aromatic or large side chains

They have fairly broad specificity

89
Q

What is the optimum pH of pepsins?

What pH are they denatured by?

A

optimum pH is around 2-3

They are denatured when pH is above 5-7, when they reach the small bowel

They are replaced by trypsins coming from the pancreas

90
Q

What is needed after pepsin to break down the peptides into individual amino acids?

A

Carboxypeptidases

They are located in the small bowel

91
Q

Why is ptyalin a-amylase present in the stomach even though it is denatured at pH4?

A

It is present in the fundus and body where there is lots of food present but it hasn’t mixed with the acid yet

It is NOT produced in the stomach - it comes from salivary glands

92
Q

How is vomiting controlled?

A

It is centrally controlled in the area postrema

This is a structure in the medulla oblongata of the brainstem

93
Q

What 5 things can cause vomiting?

A
  1. vagal afferents, in response to irritants in or around the bowel
  2. psychogenic causes, such as pain and revulsion
  3. motion sickness/labyrinthine disorders (inner-ear)
  4. drugs or toxins with a direct effect
  5. pregnancy
94
Q

What is the main consequence of vomiting?

A

Salivation, sweating and hyperventilation

Hypersalivation leads to production of very watery saliva

95
Q

What is retrograde peristalsis?

A

The loss of normal gut motility due to vomiting

There are longer peristaltic waves that work backwards from the small bowel, through the stomach and forcing relaxation of the LES

96
Q

What is retching?

A

Involuntary contractions of the diaphragm and abdominal wall muscles

97
Q

What are the other 3 consequences of vomiting?

A
  1. displacement of the cardia into the thorax
  2. reflex closure of the glottis and soft palate and opening of the LES and UES
  3. emptying of gastric (+/- small bowel) content

BS - Gastrointestinal System pt.1 (22 decks)

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